1. Field of the Invention
The present invention relates to a control circuit for DC-DC converters and a control method therefor.
2. Description of Related Art
Portable electronic equipment uses a battery, such as electric cell, as power source. The power of a battery is discharged as time passes after the portable electronic equipment is actuated, and the output voltage of the battery drops with time. DC-DC converters have been used to keep power supply voltage, supplied to portable electronic equipment, constant even after the output voltage of its battery drops.
Portable electronic equipment is connected with various devices. Therefore, it is required to prepare multiple DC-DC converters in correspondence with the target voltages of devices connected with the relevant portable electronic equipment.
To avoid preparing a separate control IC having specific characteristics for each operating condition of DC-DC converters and provide versatility, some of such portable electronic equipment are so constructed that the following is implemented: a resistance element and a capacitive element that establish operating conditions are externally added to a control IC.
Portable electronic equipment is often moved from one installation position to another when used indoors or is often taken out when used. When it is moved or taken out, it may be accidentally dropped. In such a case, troubles can be caused by dropping impact. For example, a resistance element or a capacitive element externally added to a control IC can come off or be loosened. As a result, they are not electrically connected with the control IC, or failure is produced in the electrical connection between the control IC and the resistance element or the like. As a result, a state of overvoltage in which output voltage is abruptly increased, for example, may be brought about.
Japanese Unexamined Patent Publication No. 2005-117784 discloses a technique to prevent occurrences of overvoltage. In the switching power supply unit described in this document, output voltage drop is prevented by taking the following measures: an IC for switching power supply compares reference voltage generated by a charging circuit with feedback voltage from an output circuit; and it brings the value of this feedback voltage close to the value of the reference voltage. When the IC for switching power supply is no longer electrically connected with the output circuit due to connection failure, for example, the following takes place: the value of feedback voltage from the output circuit is zeroed, and the difference between the value of feedback voltage and the value of reference voltage is increased. Consequently, the IC for switching power supply carries out control so as to bring the value of feedback voltage close to the value of reference voltage. As a result, output voltage rises, and overvoltage is caused. In this switching power supply unit, consequently, output voltage is prevented from rising by taking the following measures: when the value of feedback voltage lowers and the occurrence of a failure is detected, a switch means connected with an output terminal is turned off.
Some of the above-mentioned portable electronic equipment are provided with a DC-DC converter 100 illustrated in FIG. 7, for example. A resistor R3 and a capacitor C2 are externally added to a control circuit, and the feedback path for an error amplifier ERA1 is constructed of the resistor R3 and the capacitor C2.